High power implantable battery with improved safety and method of manufacture

ABSTRACT

A power control circuit for an implantable medical device is presented. The power control circuit includes a first high rate cell, a second high rate cell, at least one resistive load, and at least one control circuit. The at least one resistive load is connected between the first and the second high rate cells. The at least one control circuit is coupled to the first and the second high rate cells.

FIELD OF THE INVENTION

The present invention relates generally to a power source for animplantable medical device, and more particularly, the present inventionrelates to a dual cell power source for optimizing implantable medicaldevice performance.

BACKGROUND OF THE INVENTION

A variety of different implantable medical devices (IMD) are availablefor therapeutic stimulation of the heart and are well known in the art.For example, implantable cardioverter-defibrillators (ICDs) are used totreat those patients suffering from ventricular fibrillation, a chaoticheart rhythm that can quickly result in death if not corrected. Inoperation, the ICD continuously monitors the electrical activity of apatient's heart, detects ventricular fibrillation, and in response tothat detection, delivers appropriate shocks to restore normal heartrhythm. Similarly, an automatic implantable defibrillator (AID) isavailable for therapeutic stimulation of the heart. In operation, an AIDdevice detects ventricular fibrillation and delivers a non-synchronoushigh-voltage pulse to the heart through widely spaced electrodes locatedoutside of the heart, thus mimicking transthoratic defibrillation. Yetanother example of a prior art cardioverter includes thepacemaker/cardioverter/defibrillator (PCD) disclosed, for example, inU.S. Pat. No. 4,375,817 to Engle, et al. This device detects the onsetof tachyarrhythmia and includes means to monitor or detect progressionof the tachyarrhythmia so that progressively greater energy levels maybe applied to the heart to interrupt a ventricular tachycardia orfibrillation. Numerous other, similar implantable medical devices, forexample a programmable pacemaker, are further available.

Regardless of the exact construction and use, each of theabove-described IMDs generally includes three primary components: alow-power control circuit, a high-power output circuit, and a powersource. The control circuit monitors and determines various operatingcharacteristics, such as, for example, rate, synchronization, pulsewidth and output voltage of heart stimulating pulses, as well asdiagnostic functions such as monitoring the heart. Conversely, thehigh-power output circuit generates electrical stimulating pulses to beapplied to the heart via one or more leads in response to signals fromthe control circuit.

The power source provides power to both the low-power control circuitand the high-power output circuit. As a point of reference, the powersource is typically required to provide 10-20 microamps to the controlcircuit and a higher current to the output circuit. Depending upon theparticular IMD application, the high-power output circuit may require astimulation energy of as little as 0.1 Joules for pacemakers to as muchas 40 Joules for implantable defibrillators.

Suitable power sources or batteries for IMD's are virtually alwayselectrochemical in nature, commonly referred to as electrochemicalcells. Acceptable electrochemical cells for IMDs typically include acase surrounding an anode, a separator, a cathode, and an electrolyte.The anode material is typically a lithium metal or, for rechargeablecells, a lithium ion containing body. Lithium batteries are generallyregarded as acceptable power sources due in part to their high energydensity and low self-discharge characteristics relative to other typesof batteries. The cathode material is typically metal-based, such assilver vanadium oxide (SVO), manganese dioxide, etc.

IMDs have several unique power source requirements. IMDs demand a powersource with most of the following general characteristics: very highreliability, highest possible energy density (i.e., small size),extremely low self-discharge rating (i.e., long shelf life), very highcurrent capability, high operating voltage, and be hermetic (i.e., nogas or liquid venting).

These unique power source requirements pose varying battery designproblems. For example, for the heart monitoring function of an AID, itis desirable to use the lowest possible voltage at which the circuitscan operate reliably in order to conserve energy. This is typically inthe order of 1.5-3.0 V. On the other hand, the output circuit works mostefficiently with the highest possible battery voltage in order toproduce firing voltages of up to about 750 V. Traditionally, allmanufactured implantable cardioverter defibrillators used a batterysystem comprised of two cells in series to power the implantable device.This power source of about 6 volts provided improved energy efficiencyof the output circuit at the expense of energy efficiency of themonitoring circuit. However, a two-cell battery was undesirable from apackaging, cost, and volumetric efficiency perspective.

Eventually, improvements in output circuit design allowed the use of asingle 3-volt cell while still maintaining good energy efficiency. MostICDs are now designed with a single cell battery instead of dual cellsconnected in series. This approach was taken to improve the volumetricefficiency. In order to achieve the same power capability of the dualcell approach, the electrode surface area of the single cell must be atleast equivalent to the total electrode surface area of the dual cellbattery. However, the increased electrode surface area of a single cellposes a potential hazard to the IMD should an internal short circuitdevelop in the battery cell. If the electrode surface area is too high(about above 90 cm² for a Li/SVO battery) and an internal shortdevelops, the battery can get hot enough to potentially destroy the IMDselectronics and possibly burn the patient. As a result, most IMD and IMDbattery manufacturers have adopted a design rule, which limits thesurface area of a single cell to about 90 cm². This is significantlyless surface area than a typical dual cell design where the surface areawas about 130 cm². Hence, these single cell ICD batteries produced lesspower and the result was longer capacitor charge times. Many studieshave proposed that defibrillation and cardioversion shocks are mosteffective when delivered as quickly as possible following detection ofarrhythmia. The chance of terminating an arrhythmia in a patientdecreases as the length of time it takes for therapy to be delivered tothe patient increases. Therefore, the shorter the charge time for thecapacitors the more effective the defibrillation therapy. Typically,battery electrode sizes are inversely proportional to the charging time.Therefore, the quicker the desired charging time, the larger thebattery.

While single battery systems have proved workable for implantablecardioverter defibrillators, the use of a single battery systemnecessarily involves a compromise between the ideal power supply and thehazards associated with large surface area electrodes. Accordingly, itwould be desirable to provide for an improved dual battery power systemfor an implantable cardioverter defibrillator, which overcomes theproblems of earlier attempts at dual battery systems.

BRIEF SUMMARY OF THE INVENTION

Implantable medical devices in embodiments of the invention may includeone or more of the following features: (a) a hermetic enclosure, (b) alow-power control circuit located in the enclosure, (c) a high-poweroutput circuit located in the enclosure for delivering an electricalpulse therapy, (d) a power source and circuitry located in the enclosurefor powering the low-power control circuit and the high-power outputcircuit, the power source and circuitry, (e) a first high-rate cell, (f)a second high-rate cell wherein the first cell and second cell areelectrically connected in parallel to the low-power control circuit andthe high-power output circuit, (g) at least one resistive loadelectrically connected between the first high-rate cell and the secondhigh-rate cell. The resistive load posses a resistive value to prevent,in the event of an internal short in one of the high-rate cells, theshorted high-rate cell from substantially draining the other high-ratecell wherein either high rate cell is able to provide power for both thelow-power control circuit and the high-power output circuit, in theevent of a short in the other high rate cell, and (h) a switchingcircuit electrically connected between the first high-rate cell and thesecond high-rate cell for selectively coupling the first high-rate cellto the second low-rate cell upon activation of the high-power outputcircuit.

An electrochemical battery of the invention may include one or more ofthe following features: (a) a first high-rate electrochemical cellcomprising: a first anode with a first anode current collector, a firstterminal for connecting the first anode current collector to a firstexternal lead, and a first electrolyte operatively associated with thefirst anode, (b) a second high-rate electrochemical cell comprising: asecond anode with a second anode current collector; a second terminalfor connecting the second anode current collector to a second externallead; and a second electrolyte operatively associated with the secondanode, (c) a cathode electrically associated with the first electrolyteand the second electrolyte, wherein the first cell is connected inparallel to the second cell; and (d) at least one resistive loadelectrically connected between the first external lead and the secondexternal lead. Of course, the cathode could be the external lead, orboth anode and cathode could be connected to external leads.

Methods of manufacturing an electrochemical battery according to thepresent invention may include one or more of the following steps: (a)providing a first high-rate electrochemical cell, comprising the stepsof: providing a first cathode with a first cathode current collector,connecting a first external lead to the first cathode current collector,and activating the first high-rate cell with an electrolyte solutionoperatively associated with the first cathode, (b) providing a secondhigh-rate electrochemical cell, comprising the step of: providing asecond cathode with a second cathode current collector, connecting asecond external lead to the second cathode current collector, andactivating the second electrochemical cell with the electrolyte solutionoperatively associated with the second cathode, (c) associating an anodeelectrically with the electrolyte in the first high-rate cell and thesecond high-rate cell, wherein the first cell is connected in parallelto the second cell, (d) connecting at least one resistive loadelectrically between the first external lead and the second externallead and (e) connecting the anode to a battery casing to provide anegative charge on the casing.

Methods for manufacturing an implantable medical device according to thepresent invention may include one or more of the following steps: (a)providing a hermetic enclosure, (b) providing a low-power controlcircuit in the enclosure, (c) providing a high-power output circuit inthe enclosure for delivering an electrical pulse therapy, (d) providinga power source and circuitry in the enclosure for powering the low-powercontrol circuit and the high-power output circuit, (e) providing a firsthigh-rate cell, (f) providing a second high-rate cell, (g) connectingthe first cell and second cell electrically in parallel to the low-powercontrol circuit and the high-power output circuit, (h) connecting atleast one resistive load electrically between the first high-rate celland the second high-rate cell, and (i) connecting a switching circuitelectrically between the first high-rate cell and the second high-ratecell for selectively coupling the first high-rate cell to the secondlow-rate cell upon activation of the high-power output circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic view of one embodiment of animplantable medical device (IMD) incorporating a power source inaccordance with the present invention;

FIG. 2 is a simplified schematic circuit diagram of a power source inaccordance with the present invention for use with the IMD of FIG. 1;

FIG. 3 is a simplified schematic diagram of an embodiment for a powersource in accordance with the present invention;

FIG. 4 is a simplified schematic diagram of another embodiment for apower source in accordance with the present invention;

FIG. 5 is a simplified schematic diagram of another embodiment for apower source in accordance with the present invention;

FIG. 6 is a simplified schematic diagram of a high-rate dual-cellbattery embodiment in accordance with the present invention;

FIG. 7 is a simplified schematic diagram of another high-rate dual-cellbattery embodiment in accordance with the present invention;

FIG. 8 is a simplified schematic diagram of another high-rate dual-cellbattery embodiment in accordance with the present invention.

FIG. 9 is a simplified schematic diagram of another high-rate dual-cellbattery embodiment in accordance with the present invention.

FIG. 10 is a simplified schematic diagram of still yet another high-ratedual-cell battery control circuit in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description is to be read with reference to thefigures, in which like elements in different figures have like referencenumerals. The figures, which are not necessarily to scale, depictselected embodiments and are not intended to limit the scope of theinvention. Skilled artisans will recognize that the examples providedherein have many useful alternatives that fall within the scope of theinvention.

FIG. 1 is a simplified schematic view of one embodiment of animplantable medical device (“IMD”) 20 in accordance with the presentinvention and its relationship to a human heart 22. The IMD 20 is shownin FIG. 1 as preferably being a pacemaker/cardioverter/defibrillator(PCD), although the IMD may alternatively be a drug delivery device, aneurostimulator, or any other type of implantable device known in theart. The IMD includes a case or hermetic enclosure 23 and associatedelectrical leads 24, 26, and 28. As described in greater detail below,the enclosure case 23 contains various circuits and a power source. Theleads 24, 26 and 28 are coupled to the IMD 20 by means of a multi-portconnector block 30, which contains separate ports for each of the threeleads 24, 26, and 28 illustrated.

In one embodiment, lead 24 is coupled to a subcutaneous electrode 40,which is intended to be mounted subcutaneously in the region of the leftchest. Alternatively, an active “can” may be employed such thatstimulation is provided between an implanted electrode and enclosurecase 23. In yet another embodiment, stimulation is provided between twoelectrodes carried on a single multipolar lead.

The lead 26 is a coronary sinus lead employing an elongated coilelectrode that is located in the coronary sinus and great vein region ofthe heart 22. The location of the electrode is illustrated in brokenline format at 42, and extends around the heart 22 from a point withinthe opening of the coronary sinus to a point in the vicinity of the leftatrial appendage.

Lead 28 is provided with an elongated electrode coil 38, which islocated in the right ventricle of the heart 22. The lead 28 alsoincludes a helical stimulation electrode 44, which takes the form of anextendable/retractable helical coil, which is screwed into themyocardial tissue of the right ventricle. The lead 28 may also includeone or more additional electrodes for near and far field electrogramsensing.

In the system illustrated, cardiac pacing pulses are delivered betweenthe helical electrode 44 and the coil electrode 38. The electrodes 38and 44 are also employed to sense electrical signals indicative ofventricular contractions. Additionally, cardioverters/defibrillationshocks may be delivered between coil electrode 38 and the electrode 40,and between coil electrode 38 and electrode 42. During sequential pulsedefibrillation, it is envisioned that pulses would be deliveredsequentially between subcutaneous electrode 40 and coil electrode 38,and between the coronary sinus electrode 42 and coil electrode 38.Single pulse, two electrode defibrillation pulse regimens may also beprovided, typically between coil electrode 38 and the coronary sinuselectrode 42. Alternatively, single pulses may be delivered betweenelectrodes 38 and 40. The particular interconnection of the electrodesto the IMD 20 will depend somewhat on the specific single electrode pairdefibrillation pulse regimen is believed more likely to be employed.

Regardless of the exact configuration and operation of the IMD 20, theIMD 20 includes several basic components, illustrated in block form inFIG. 2. The IMD 20 includes a high-power output circuit 50, a low-powercontrol circuit 52, a power source 54 (shown with dashed lines), andcircuitry 56. As described in greater detail below, the power source 54is preferably a dual-cell configuration, and can assume a wide varietyof forms. Similarly, the circuitry 56 can include analog and/or digitalcircuits, can assume a variety of configurations, and electricallyconnects the power source 54 to the high power circuit 50 and thelow-power circuit 52.

The high-power output circuit 50 and the low-power control circuit 52are typically provided as part of an electronics module associated withthe IMD 20. In general terms, the high-power output circuit 50 isconfigured to deliver an electrical pulse therapy, such as adefibrillation or a cardioversion/defibrillation pulse. In sum, thehigh-power output circuit 50 is responsible for applying stimulatingpulse energy between the various electrodes 38-44 (FIG. 1) of the IMD20. As is known in the art, the high-power output circuit 50 may beassociated with a capacitor bank (not shown) for generating anappropriate output energy, for example in the range of 0.1-40 Joules.

The low-power control circuit 52 is similarly well known in the art. Ingeneral terms, the low-power control circuit 52 monitors heart activityand signals activation of the high-power output circuit 50 for deliveryof an appropriate stimulation therapy. Further, as known in the art, thelow-power control circuit 52 may generate a preferred series of pulsesfrom the high-power output circuit 50 as part of an overall therapy.

The power source 54 and associated circuitry 56 can assume a widevariety of configurations, as described in the various embodimentsbelow. Preferably, however, the power source 54 includes a first,high-rate cell 60, and a second, high-rate cell 62. However, it is fullycontemplated that power source 54 could contain a plurality of high-ratecells within volumetric reason so that IMD 20 does not become to largefor implantation or uncomfortable to the patient. Notably the first andsecond cells 60, 62 can be formed separate from one another or containedwithin a singular enclosure. However, as is discussed below, preferablycells 60, 62 are contained within a singular enclosure. First and secondcells can 60, 62 can have any amount of electrode surface area withinreason to deliver the proper amount of surface energy. However,preferably cells 60, 62 have an electrode surface area of between 45 cm²and 90 cm² each to provide high power output. Depending upon theparticular application, high-rate cells 60, 62 are configured to providea stimulation energy of as little as 0.1 Joules for pacemakers to asmuch as 40 Joules for implantable defibrillators. As described belowwith reference to specific embodiments, high-rate cells 60, 62 canassume a wide variety of forms as is known in the art. Preferably,high-rate cells 60, 62 include an anode, a cathode, and an electrolyte.The anode is preferably formed to include lithium, either in metallicform or ion form for re-chargeable applications. With this in mind,high-rate cells 60, 62 are most preferably a spirally wound battery ofthe type disclosed, for example, in U.S. Pat. No. 5,439,760 to Howard etal. for “High Reliability Electrochemical Cell and Electrode AssemblyTherefor” and U.S. Pat. No. 5,434,017 to Berkowitz et al. for “HighReliability Electrochemical Cell and Assembly Therefor,” the disclosuresof which are hereby incorporated by reference. High-rate cells 60, 62may less preferably be a battery having a spirally wound, stacked plate,or serpentine electrodes of the type disclosed, for example, in U.S.Pat. Nos. 5,312,458 and 5,250,373 to Muffuletto et al. for “InternalElectrode and Assembly Method for Electrochemical Cells;” U.S. Pat. No.5,549,717 to Takeuchi et al. for “Method of Making Prismatic Cell;” U.S.Pat. No. 4,964,877 to Kiester et al. for “Non-aqueous Lithium Battery;”and U.S. Pat. No. 5,14,737 to Post et al. for “Electrochemical Cell WithImproved Efficiency Serpentine Electrode;” the disclosures of which areherein incorporated by reference.

Materials for the cathode of high-rate cells 60, 62 are most preferablysolid and comprise as active components thereof metal oxides such asvanadium oxide, silver vanadium oxide (SVO) or manganese dioxide, as isknown in the art. Alternatively, the cathode for high-rate cells 60, 62may also comprise carbon monofluoride and hybrids thereof or any otheractive electrolytic components and combination. Where SVO is employedfor the cathode, the SVO is most preferably of the type known as“combination silver vanadium oxide” (or “CSVO”) as disclosed in U.S.Pat. Nos. 5,221,453; 5,439,760; and 5,306,581 to Crespi et al, althoughother types of SVO may be employed.

It is to be understood that electrochemical systems other than those setforth explicitly above may also be utilized for high-rate cells 60, 62,including, but not limited to, anode/cathode systems such aslithium/silver oxide; lithium/manganese oxide; lithium/V₂O₅;lithium/copper silver vanadium oxide; lithium/copper oxide; lithium/leadoxide; lithium/carbon monofluoride; lithium/chromium oxide;lithium/bismuth-containing oxide; lithium/copper sulfate; mixtures ofvarious cathode materials listed above such as a mixture of silvervanadium oxide and carbon monofluoride; and lithium ion rechargeablebatteries, to name but a few.

With the above-described parameters of high-rate cell 60 and high-ratecell 62 in mind, one preferred combination of a power source 54A andcircuitry 56A is depicted schematically in FIG. 3. The power source 54Aincludes high-rate cell 60A and high-rate cell 62A as described above.Unlike U.S. Pub. No. 2002/0183801 A1 herein incorporated in its entiretyby reference, which includes a high-rate and low-rate cell selectivelyconnected in parallel, circuitry 56A electrically connects high-ratecell 60A and high-rate cell 62A in parallel to high-power output circuit50 and low-power control circuit 52. In particular, the circuitry 56Aincludes a switch 70 configured to selectively couple high-rate cells60A and 62A to high-power control circuit 50. In this regard, circuitry56A can include additional components/connections (not shown) foractivating and deactivating switch 70 in response to operationalconditions described below. Circuitry 56A further includes resistiveload 64 (e.g. a resistor etc.) to limit the current delivered from anon-shorted cell to a shorted cell in the event of an internal shortwithin one of cells 60A or 62A.

This battery circuit design allows two high-rate cells to be connectedin parallel to achieve the same power capability as two cells connectedin series. Resistor 64 is selected such that R≧10R_(Cell) where R is theresistance of resistive load 64 and R_(Cell) is the effective resistanceof a short in high-rate cell 60 or 62. However, preferably R≧100R_(Cell)and R<<10R_(Circuit) where R_(Circuit) is the input impedance oflow-power circuitry 52. Generally, R can be any reasonable value withinthe specifications above, but preferably R is between 10-100 ohms andR_(Cell) is about 0.5 ohm. This resistive relationship allows both cells60A and 62A to be discharged uniformly under pacing and sensingconditions, which is described in more detail below. Additionally,skilled artisans appreciate that a high rate cell is defined asincluding a R_(Cell) that is less than 0.5 ohm.

In normal operation, switch 70 is open until it is necessary to delivera defibrillation pulse and then the switch is closed. Switch 70 isselected such that R_(Switch)<<R_(charge)<<R, RCHARGE where R_(Charge)is the input impedance of high-power circuitry 50. Switch 70 is closedonly when charging a defibrillation capacitor (not shown) and would beenabled only when the voltage across load 64 was below a pre-determinedvalue of about 20 millivolts indicating that neither cell 60A nor 62Ahas an internal short. If switch 70 was enabled when either cell 60A or62A had an internal short, then the current from the non-shorted cellwould dissipate into the shorted battery and would quickly deplete bothcells, create enough heat to damage circuitry, and possibly causediscomfort the patient. In an alternative embodiment load 64 could besubstituted with a fuse.

This power source/circuitry configuration provides a distinct advantageover prior art, single-cell and dual-cell in series designs. The primaryadvantage is two high-rate cells can be assembled in parallel in thesame enclosure. This is generally 20% more volumetrically efficient thantwo cells in series. Further, the risk of damage to the IMD and harm tothe patient is substantially reduced. Another advantage of the presentinvention is that it allows single cell electronic circuits to beretrofitted to a parallel two-cell design with significantly minimalcircuit design changes. During operation of the IMD 20 (FIG. 1), thepower source 54A is, from time-to-time, required to deliver ahigh-current pulse or charge to high-power output circuit 50 whilemaintaining a voltage high enough to continuously power low-powercontrol circuit 52. If the supply voltage drops below a certain value,the IMD 20 will cease operation. This power source/circuitryconfiguration places the high-rate cells 60A and 62A in parallel topower both low-power control circuit 52 and when necessary high-powercircuit 50. During a transient high power pulse, such as adefibrillation pulse, the switch 70 is operated to couple high-rate cell60A with high-rate cell 62A with minimal resistance and thereforesubstantially all the power from cells 60A and 62A is transferred tohigh-power circuit 50. The low battery resistance provided by theparallel combination of cells 60A and 62A prevents an excessive voltagedrop during a transient high power pulse and assures continuousoperation of low-power circuit 50. Further, where desired, the cells 60Aand/or 62A can be sized and shaped to satisfy certain volumetric orshape constraints presented by the IMD 20 (FIG. 1).

With reference again to FIG. 3, if an internal short were to occur, forexample, within cell 60A and resistive load 64 were not in circuit 56A,then cell 62A would begin to discharge into cell 60A until cell 62A wasdepleted beyond usefulness. Similarly, if an internal short were tooccur, for example, within cell 62A and resistive load 64 were not incircuit 56A, then cell 62A would begin to discharge into cell 60A untilcell 60A was depleted beyond usefulness.

This would make IMD 20 unable to provide therapeutic stimulation andthus IMD 20 would have to be explanted and another IMD implanted.Further, the short would create a lot of heat, which could destroy theelectronics of the IMD and cause potentially serious discomfort to thepatient. However, with resistive load 64 between cell 60A and 62 A incircuit 56A, cell 62A is limited in the amount of power that can bedelivered to shorted cell 60A due to the parallel construction.

The parallel battery construction of the present invention allows cells60A and 62A to deplete at an equal rate over the life of IMD 20. Forexample, when a defibrillation pulse is needed, switch 70 is closed,after it is determined that there is no internal short in cells 60A or62A, and cells 60A and 62A begin discharging into high-power circuit 50.The only difference in the current path between cell 60A and 62A is thatthe current path for cell 62A must travel through the resistance ofswitch 70. It's of note that the current path is generally throughswitch 70 and not resistor 64 since current will take the path of leastresistance. Therefore, since R_(switch) has a lower value than load 64,the current path from cell 62A will be through switch 70. Since switch70 has a small resistance, cell 60A and 62A will deplete at asubstantially equal rate during defibrillation pulses since there is aminimal voltage drop at switch 70.

In a similar fashion, when cells 60A and 62A are powering low-powercircuit 52 the only difference in the current path between cell 60A and62A is that the current path for cell 60A must travel through load 64.Since load 64 has a relatively small resistance and the currenttraveling through load 64 is between 10-20 microamps, then the voltagedrop at load 64 is extremely low, about between 0.1 and 2 millivolts,and therefore cell 60A and 62A will deplete at a substantially equalrate while supplying low-power circuit 52.

With reference to FIG. 4, another embodiment for a power source isshown. The circuit 20 is substantially the same as the circuit in FIG.3, except that switches 68 and 66 have been added to circuit 56B. Thecircuit 20 of FIG. 4 allows for one of cells 60B and 62B to become abackup should the other one experience an internal short. For example,normally switches 68 and 66 are closed to provide normal operation ofthe circuit 20. However, should a short occur in either cell 60B or cell62B, a voltage difference would occur across resistor 64. The polarityof that voltage will determine which cell is shorted and that switchwill be opened. For example, if the negative terminal of resistor 64 isclosest to cell 60B, cell 60B is shorted or depleted and switch 66 isopened. Similarly, if the negative terminal of resistor 64 is closest tocell 62B, cell 62B is shorted or depleted and switch 68 is opened. Thevoltage difference will be slight at first but will increase. The shortwill draw both cells 60B, 62B down. If cell 60B shorts with resistanceR, current in 60B would be about

$\frac{V_{60\; B}}{R}$

and the current in 62B would be about

$\frac{V_{62\; B} - V_{60\; B}}{R}$

(where V_(60B) and V_(62B) are the cell voltages). Therefore, cell 60Bwould deplete faster than cell 62B. Once the voltage difference reachesabout X number volts (e.g. 0.2 volts etc.), the correct switch would beopened and some method of notifying the patient or physician would beactivated. A small hysteresis would be built in to the comparator so asto avoid possible chatter from loading and unloading the shorted cell.

The circuit is substantially the same as the circuit in FIG. 3, exceptthat switches 68 and 66. have been added to circuit 56B. The circuit ofFIG. 4 allows for one of cells 60B and 62B to become a backup should theother one experience an internal short. Since each cell 60B and 62B is ahigh-rate cell, IMD 20 is able to function normally, except for a slowerdefibrillation capacitor-charging time, until cell 50B becomes depletedenough and explanting is necessary. One additional embodiment associatedwith FIG. 4. On rare occasion the failure of a circuit component caneffectively short circuits the battery. The heat generated during thisfailure mode can cause significant discomfort to the patient. With thedesign shown in FIG. 4, an external short would also show up as avoltage drop across load 64. The same algorithm described forces onecell to effectively disconnect, thereby greatly reducing the rate ofenergy dissipation due to the short.

With reference to FIG. 5, another embodiment for a power source isshown. The circuit is similar to the circuit of FIG. 3 except thatswitch 70 has been removed. This embodiment still protects IMD 20 froman internal short, however, this embodiment is much more inefficientwhen operating in a defibrillating mode. This is because the currentfrom cell 60C must travel through load 64. This creates a large voltagedrop at load 64 and thus it takes longer to fully charge thedefibrillation capacitor.

With reference to FIG. 6, a simplified schematic of a high-rate dualcell battery is shown. In this embodiment power source 54 is shownhaving battery case 72, anode 74, cathode 76, cathode 77, separator 86,feedthrough 84, feedthrough 82, terminal 78, and terminal 80. Batterycase 72 can be variable in shape and construction. Battery case 72 canbe a deep drawn case as discussed in U.S. Pat. No. 6,040,082 (Haas et.al.) herein incorporated in its entirety by reference or a shallow drawncase as discussed in U.S. patent application Ser. No. 10/260,629attorney docket number P-10765.00 filed on Sep. 30, 2002 titledContoured Battery for Implantable Medical Devices and Method ofManufacture herein incorporated in its entirety by reference. Batterycase 72 is preferably made of a medical grade titanium, however, it iscontemplated that battery case 72 could be made of almost any type ofmaterial, such as aluminum and stainless steel, as long as the materialis compatible with the battery's chemistry in order to preventcorrosion. Further, it is contemplated that battery case 72 could bemanufactured from most any process including but not limited tomachining, casting, thermoforming, or injection molding.

In the embodiment of FIG. 6, one electrode 74 is continuous and isconnected to case 72. The alternate electrode is in two separate pieces76 and 77. Each piece 76 and 77 has a separate electrical lead 78 and 80through a feedthroughs 84 and 82 respectively that is electricallyisolated from case 72. It is contemplated that battery 54 can be casenegative (anode connected to case) or case positive (cathode connectedto case). As shown, dual cell battery 54 has one anode 74, which isutilized by a first cell chamber 88 and a second cell chamber 90, whichare separated by separator 86. There is no requirement of a hermeticseal between cells 88 and 90. They could be designed this way, but itwould be an unnecessary complication and result in a decrease involumetric efficiency. Separator 86 is used to prevent direct electricalcontact between anode 74 and cathodes 76 and 77. It is a porous materialthat allows transport of electrolyte ions. Li/SVO batteries typicallyuse separators comprised of porous polypropylene or polyethylene, butthere are many other materials used for other battery chemistries.Nevertheless, separator 86 is not required for the present invention andcan and power source 54 can operate without it. Further, it is notedthat the anode/cathode relationship could be reversed. For example,anode 74 could be replaced with a cathode as long as cathodes 76 and 77were switched to anodes. It is understood that the orientation of theanodes and cathodes is not a critical aspect of the invention. Althoughlithium hexafluoroarsenate is preferably used in both cells 88 and 90for the present embodiment, it is contemplated that most any chemicalelectrolyte could be used without departing from the spirit of theinvention for either cell chamber 88 or 90. Cathodes 76 and 77 arelocated within cells 88 and 90 respectively and are connected toexternal leads 78 and 80 respectively, which traverse out of batterycase 72 through feedthroughs 84 and 82. While power source 54 is shownwith two feedthroughs, it is fully contemplated that battery case 72could have one feedthrough to accommodate both leads 78 and 80. Finally,resistive load 92 is shown connected between leads 78 and 80. Asdiscussed above, load 92 functions to limit the amount of powerdelivered from a non-shorted cell to a shorted cell.

With reference to FIG. 7, a simplified schematic of another high-ratedual cell battery is shown. In contrast to the dual cell embodiment ofFIG. 6, continuous electrode 74 is not connected to case 72. Insteadelectrical lead 79 extends through feedthrough 89 to make case 72neutral.

With reference to FIG. 8, a simplified schematic of another high ratedual battery is shown. In contrast to the dual cell embodiment of FIG.6, the anode is not continuous and each piece 74 and 75 is connected tocase 72. This would be equivalent to taking two completely separatecells and placing them in the same battery case.

With reference to FIG. 9, a simplified schematic of another high ratedual battery is shown. This design is similar to the embodiment of FIG.8, except electrical leads 96 and 98 traverses through feedthroughs 92and 94 to make a case neutral design.

FIG. 10 is a simplified schematic of a high rate dual battery powercontrol circuit 200. Circuit 200 includes high rate cells 60B, 62B,comparators 204A, 204B, switches 66, 68, and 70, resistor 64, and atleast one control circuit 202. Circuit 202 includes high-power outputcircuit 50 and low-power control circuit 52. Each comparator 204A, 204Bincludes an off-set. In this embodiment, typically both cells 60B, 62Bare operational, which means a first and a second switch 66, 68 aretypically closed and the output from comparators 204A and 204B is zero.If cell 60B becomes shorted or depleted, then the output of comparator204A goes HIGH (e.g. 3 volts etc.) and switch 66 is opened. If cell 62Bbecomes shorted or depleted, then the output of comparator 204B goesHIGH and switch 68 is opened.

If the battery shares electrolyte (or a common electrode), the cells donot directly short if one of the cells has an internal short because inorder to have such an internal short, two conditions are required.First, there must be a direct electrical connection between an anode anda cathode. Second, there must be an ionic pathway between theelectrically connected anode and cathode in order to have a completecircuit. In our examples, the second condition is present, but not thefirst. For the historical method of connecting two entirely separatecells in series, we have the first condition, but not the second. Thus,it would be impossible to place two cells in series in the sameenclosure (with a common electrolyte) because both conditions are metand the cells would short. In a parallel configuration, however, it ispossible to enclose them with the same electrolyte because there is noelectrical pathway between the anode and cathode.

It will be appreciated that the present invention can take many formsand embodiments. The true essence and spirit of this invention aredefined in the appended claims, and it is not intended that theembodiment of the invention presented herein should limit the scopethereof.

1. An implantable medical device comprising: a hermetic enclosure; atleast one control circuit located in the enclosure; a first high-ratecell; a second high-rate cell; and at least one resistive loadelectrically connected between the first high-rate cell and the secondhigh-rate cell.
 2. The implantable medical device of claim 1, whereinthe at least one resistive load value is between 10 ohms and 100 ohms.3. An implantable medical device of claim 1, wherein the at least onecontrol circuit includes a high power control circuit and a low powercontrol circuit.
 4. The implantable medical device of claim 3, whereinthe electrode surface area being between 65 cm² and 90 cm².
 5. Theimplantable medical device of claim 1, wherein the first high-rate celland the second high-rate cell are maintained within a single case. 6.The implantable medical device of claim 1, wherein the first high-ratecell and the second high-rate cell includes a cathode, and furtherwherein the first high-rate cell and the second high-rate cell share acommon anode.
 7. The implantable medical device of claim 1, wherein thefirst high-rate cell and the second high-rate cell includes an anode,and further wherein the first high-rate cell and the second high-ratecell share a common cathode.
 8. The implantable medical device of claim7, wherein the cathode is selected from the group consisting of sliveroxide, vanadium oxide, silver vanadium oxide, manganese dioxide, copperoxide, copper silver vanadium oxide, lead oxide, carbon monoflouride,chromium oxide, bismuth-containing oxide, copper sulfate, and mixturesthereof.
 9. The implantable medical device of claim 8, wherein the anodeof the first high-rate cell and the anode of the second high-rate cellare formed from lithium.
 10. The implantable medical device of claim 1,further comprising a switching circuit electrically connected betweenthe first high-rate cell and the second high-rate cell for selectivelycoupling the first high-rate cell to the second low-rate cell uponactivation of the high-power output circuit.
 11. An electrochemicalbattery for an implantable medical device comprising: a) a firsthigh-rate electrochemical cell comprising: i) a first anode; ii) a firstterminal for connecting the first anode to a first external lead; iii) afirst electrolyte operatively associated with the first anode; and b) asecond high-rate electrochemical cell comprising: i) a second anode; ii)a second terminal for connecting the second anode to a second externallead; iii) a second electrolyte operatively associated with the secondanode; and c) a cathode electrically associated with the firstelectrolyte and the second electrolyte, wherein the first cell isconnected in parallel to the second cell; and d) at least one resistiveload electrically connected between the first external lead and thesecond external lead.
 12. The electrochemical battery of claim 11wherein the first and second high-rate cells discharge at a generallyequal rate.
 13. The electrochemical battery of claim 11 wherein thefirst high-rate cell and the second high-rate cell are housed in a onecasing.
 14. The electrochemical battery of claim 11 wherein the firstelectrolyte and the second electrolyte are the same electrolyte.
 15. Theelectrochemical battery of claim 11 wherein the at least one resistiveload has a resistive value to reduce, in the event of an internal shortin one of the high-rate cells, the rate at which the shorted high-ratecell drains the other high-rate cell.
 16. A method for manufacturing animplantable medical device comprising: a) providing a hermeticenclosure; b) providing at least one control circuit; and c) providing apower source and circuitry in the enclosure for powering the low-powercontrol circuit and the high-power output circuit, comprising: i)providing a first high-rate cell; ii) providing a second high-rate cell;and iii) connecting at least one resistive load electrically between thefirst high-rate cell and the second high-rate cell.
 17. A power controlcircuit for an implantable medical device comprising: a first high ratecell; a second high rate cell; at least one resistive load connectedbetween the first and the second high rate cells; and at least onecontrol circuit coupled to the first and the second high rate cells. 18.The circuit of claim 17 wherein the at least one resistive load being aresistor.
 19. The circuit of claim 17 wherein the at least one resistiveload not being line resistance.
 20. The circuit of claim 17 furthercomprising: a first comparator coupled to the first high rate cell; anda second comparator coupled to the second high rate cell.
 21. Thecircuit of claim 17 wherein a first offset associated with the firstcomparator and a second offset associated with the second comparator.22. The circuit of claim 17 wherein a first switch associated with thefirst high rate cell and a second switch associated with the second highrate cell.
 23. The circuit of claim 17 wherein the first high rate cellbeing defined as including a resistive load (Rcell) being less than 0.5ohms.